Asymmetry of life may have cosmic origin

Posted January 10, 2011 - 05:34
by
Emma Woollacott

The asymmetry of biological molecules may have come from space, say French scientists.

So-called chiral molecules, including amino acids and sugars, can exist in two forms which are mirror images of each other. However, here on Earth, they exist in only one form, either left-handed or right-handed.

For instance, the amino acids that make up proteins only exist in one of their two enantiomeric forms, the left-handed form. On the other hand, the sugars present in the DNA of living organisms are solely right-handed. The phenomeon is known as homochirality.

In a study, the team reproduced the conditions found in interstellar space and found that there, too, biological molecules tend towards one form or the other.

One theory is that life originated from a mixture containing 50 percent of one type and 50 percent of the other, and that homochirality progressively emerged during the course of evolution.

Another is that asymmetry leading to homochirality preceded the appearance of life and was of cosmic origin. This is supported by the predominance of left-handed molecules in certain amino acids extracted from primitive meteorites.

The researchers first reproduced analogs of interstellar and cometary ices in the laboratory. Using the DESIRS beamline at the SOLEIL synchrotron facility, the ices were subjected to circularly polarized ultraviolet radiation, reckoned to mimic the conditions encountered in some space environments.

When the ices were warmed up, an organic residue was produced - and tests showed a bias towards one particular handedness comparable to that measured in primitive meteorites.

The result, they say, reinforces the hypothesis that the origin of homochirality is prebiotic and interstellar. According to their scenario, the asymettry of life's molecules is derived from the delivery of extraterrestrial organic material. This material may even have formed outside the solar system, they say.